Interaction of dibucaine with lipids in mixed Langmuir monolayers

The direct incorporation of dibucaine (DIB) into dipalmitoylphosphatidylcholine (DPPC) monolayers was investigated using surface pressure, surface potential and lateral conductance measurements. DPPC + DIB mixed monolayers became increasingly more condensed with increasing concentration of DIB, the change in area occurring because DIB molecules occupy a smaller area. The close packing of the DPPC monolayers was not affected and the overall shape of the pressure-area isotherm was retained. Surface potential data provided evidence that the aromatic part of DIB lies close to the border between the polar heads and the hydrophobic tails of DPPC with the tertiary amino group facing the subphase. This was consistent with previous studies involving the introduction of DIB into lipid vesicles. Enhanced conductance was observed at the critical area at which the surface potential started to increase. The lateral conductance of the mixed films was smaller than that for pure DPPC, as the charged amino group of DIB interacts with the phosphate group of DPPC, preventing the latter from acting as a site for proton hopping.